Holocene sedimentation at Cape Sable, south Florida

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A regionally distinct mosaic of sedimentary environments including beaches and beach ridges which formed under relatively high-energy conditions through low energy, tide-dominated environments to quiescent inland lakes and ponds, exists at Cape Sable, south Florida. Environments of deposition from the sandy capes inland are: (1) shell beaches and beach ridges; (2) black mangrove mudflats; (3) ponded mudflats; (4) exposed mudflats; (5) Lake Ingraham; (6) coastal levees and supratidal plain; and (7) ephemeral ponds. With exception of the beaches and beach ridges, sediments of the other environments are dominantly calcareous muds and silts and represent a depositional history related to frequency of tidal inundation and storms. Of the three capes exposed roughly from northwest to southeast at Cape Sable, radiocarbon dating shows that Northwest Cape formed in its present position or possibly migrated to it 1980 ± 100 years B.P., while Middle Cape and East Cape data from 1610 ± 100 years B.P. and 1230 ± 95 years B.P. Relict shoreline features, coastal levees, on the supratidal plain indicate an episode of coastal progradation which started 2280 ± 100 years B.P. and apparently ended about 1560 ± 80 years B.P. The entire column of Holocene carbonate sediments in the cape area rests on a thin, intermittent basal peat which dates 4950 ± 120 years B.P. Cores through the supratidal plain reveal a basal carbonate-mud sequence much like subtidal sediments of modern Florida Bay. This unit has a typical marine carbonate-mineral suite, numerous shallow-marine molluscs, and an abundance of Thalassia root burrows. An intermediate intertidal unit characterized by marine brackish carbonate sediments with algal laminations is overlain by massive aragonitic supratidal silts and silty clays. The entire sequence is capped by a dark, highly organic zone representative of the coastal levee and intervening algal-flat sediments. Trace quantities of dolomite and substantial amounts of low-Mg calcite found throughout the cores are considered to be of detrital origin. Localized surface concentrations of dolomite associated with the coastal levees may have a mixed in-situ and detrital origin. Investigation of the extractable organic matter in the intertidal facies of cores through the three prominent coastal levees indicates that diagenesis of organic matter has systematically occurred from the youngest to the oldest levee. Fatty alcohols and total hydrocarbons increase with time while fatty acids decrease. Partial decarboxylation and chemical reduction of fatty acids can occur within the time interval of 750 years between the oldest and youngest coastal levees. Geochemistry of interstitial water from the supratidal-plain cores indicated a general increase in chlorinity from the youngest to the oldest coastal levee. Chlorinity levels drop at the base of all three cores analyzed which may be related to fresher groundwater influence from the Miami Limestone. Ratios of calcium, magnesium, and strontium to chloride are remarkably constant throughout the cores. Higher relative strontium concentrations in the core from the intermediate coastal levee may indicate a subtle dissolution of aragonite. In general, evidence of diagenesis in these carbonate sediments is not readily apparent. © 1977.

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Sedimentary Geology

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